Double grating three dimensional spectrograph

ABSTRACT

A spectrograph with a first concave spectrographic diffraction grating is positioned to receive light from the input light source is configured to provide a diffracted light output dispersing the components of the input light source in a first direction. The dispersion forms the input light into an intermediate spectra. The intermediate spectra is formed in a focal surface by the once diffracted light. A slit is substantially positioned on the focal surface. A second concave diffraction grating is positioned to receive once diffracted light from the slit and configured to provide a twice diffracted light output, the second concave diffraction grating dispersing the components of the input light source in a second direction. The second direction is different from the first direction, the dispersion forming the input light into an output spectra.

REFERENCE TO GOVERNMENT FUNDING

Not Applicable.

TECHNICAL FIELD

This invention relates to diffraction-grating spectrographs and, moreparticularly, to a three dimensional, double diffraction gratingspectrograph.

BACKGROUND OF THE INVENTION

It is known in the art to use various optical modules that are assembledinto a desired configuration to perform a single specified opticalfunction. Such configuration may take the form of a scientificinstrument, or may find employment in a commercial spectroscopyapplication. It is usually advantageous to make each module as compactas possible.

A spectroscope, an instrument which produces a spectrum from inputlight, is one particularly useful example of such an optical instrument.The term “spectrum” is meant to encompass less than a complete range ofwavelengths, for example, the collection of wavelengths emitted by asample under excitation or passed by a sample filtering input lightenergy. Another example is a spectrograph which is a spectroscopeprovided with a recording device, or other light-capture means, or thelike to receive and record or otherwise process the spectrum generated.The present invention relates to an optical arrangement particularlyuseful in spectrographs, and that term will be used hereinafter. Howeverit will be understood that the novel optics described herein can beemployed in spectroscopes or spectrometers and generally allapplications where recording, quantification or similar capabilities arerequired and the invention extends to such novel spectroscopic andspectrometric applications employing the inventive optics. To the extentthat the invention may be applied to output a single spectral band or toprovide a scanned output comprising a series of individual spectralbands, the term “spectrometer” should also be understood to includemonochromators.

Diffraction grating spectrographs use one or more diffraction gratingsto diffract input light into a spectrum of specific wavelengths orspectral bands.

In a typical configuration, spectrographs are designed to select asingle wavelength, or a narrow spectral band from the input light, forexamination or recordal.

In one known embodiment of spectrometer employing a planar diffractiongrating, a concave mirror is illuminated by a point source whosespectrographic composition is to be analyzed. The light from the pointsource is collimated by the concave mirror to form a parallel bundle ofrays, which are caused to fall upon the surface of a planar diffractiongrating. This concave mirror is known as a collimator in a typicalspectrometer instrument. Because the planar diffraction grating has anumber of grooves etched in its surface, light falling on the surface ofthe diffraction grating is diffracted, that is, reflected at an anglewhich is a function of the wavelength of the light. If the input lightsource comprises a number of wavelengths, the result is that light ofdifferent wavelengths will be diffracted, or reflected, at an anglewhich is a function of wavelength.

The diffracted light may then be received by a second concave mirrorwhich focuses the diffracted light to form an image of the point sourceunder analysis. However, because light of different wavelengths has beendiffracted at different angles, the point source is imaged by the secondconcave mirror, also known as a focusing mirror, at different points fordifferent wavelengths. Accordingly, it is possible to select outindividual wavelengths, or more precisely a narrow region of thespectrum, or spectral band, consisting essentially of a singlewavelength, to measure the intensity of the same and to utilize thisinformation, for example for elemental analysis of an emissive sourcematerial.

Spectrometric elemental analysis of samples has many industrial uses.For example, in the case of the analysis of industrial slag, such asmight be obtained from a crucible filled with molten metal in a steelfurnace, the slag may be exited into a plasma, and the emission spectrumanalyzed and measured with a spectrometer. The wavelengths appearing inthe plasma emission spectra indicate the nature and quantity of theimpurities in the slag, enabling plant operators to adjust productionparameters to achieve a desired product.

While the above discussion has centered on spectrometer devices usingmirrors, and such devices are usually preferred because of the qualityof imaging using mirrors, it is possible to construct devices usingfocusing lenses, such as convex lenses or compound multielement lenseshaving an overall convex optical characteristic. In principle, it isalso possible to combine lenses and mirrors in an instrument.

It is also noted that diffraction gratings in spectrometers may beeither classical mechanically ruled diffraction gratings of the typeinvented and made by applicant's assignee at the beginning of the1800's, or holographic diffraction gratings of the type manufactured byapplicant's assignee since the 1960's.

It is also known that spectrometers may be constructed using concavediffraction gratings, such as concave holographic diffraction gratingsof the type invented by Flamand in the late 1960's working at theapplicant company as illustrated by his U.S. Pat. No. 3,628,849.

A Littrow-mounted system is a relatively common method of utilizinglarge plane reflection gratings, providing simplicity and good opticalquality arising from the use of a single mirror to perform bothcollimating and focusing functions. Moreover, in this configuration, thecollimating and focusing functions are both performed in the samegeometric space, resulting in efficient use of that space. In a typicalLittrow setup, a mirror delivers parallel incident light from an inputpoint source to the grating, and focuses diffracted light received fromthe grating to an output point often proximate the input point source.In such devices, a single mirror acts as both collimator and focusingelement at once, minimizing the number of optical elements required.

In addition to its simplicity, employing the Littrow configuration isparticularly desirable for its high quality output. Because the inputand output light beams traverse the same optical path, in oppositedirections, optical aberrations in the collimating and focusingcomponents are auto-corrected, or self compensating, so that imagequality is diffraction limited, i.e. limited by the physical propertiesof the optical system not by the deficiencies of the optics.

A particular drawback of such conventional Littrow-mounted gratingconfigurations is the difficulty and expense of providing a grating witha central optical opening. Another drawback is that undue stray lightmay be returned to the aperture by the mirror. It would be desirable toprovide a spectrometer or comparable optical system, which did notsuffer from these drawbacks.

SUMMARY OF THE INVENTION

A spectrograph providing a light output in a selected spectral band froma sample light input comprises an input light source. A first concavespectrographic diffraction grating is positioned to receive light fromthe input light source and is configured to provide a diffracted lightoutput dispersing the components of the input light source in a firstdirection. The dispersion forms the input light into an intermediatespectra. The intermediate spectra is formed in a focal surface by theonce diffracted light. A slit is substantially positioned on the focalsurface or approximately on the focal surface to accommodate amechanical member may be movably mounted to vary the position of theslit. The slit has a first edge whose position corresponds to theshortest wavelength for light to be passed by the slit and has a secondedge whose position corresponds to the longest wavelength for light tobe passed by the slit. A second concave diffraction grating ispositioned to receive once diffracted light from the slit and configuredto provide a twice diffracted light output, the second concavediffraction grating dispersing the components of the input light sourcein a second direction. In the preferred embodiment, the second directionis perpendicular to the first direction, the dispersion forming theinput light into an output spectra. The output spectra is formed on afocal surface by the twice diffracted light. A measuring device ispositioned substantially along the focal plane formed by the secondconcave diffraction grating. The measuring device may be a solid-statedetector array.

In an alternative embodiment, the first diffraction grating can bemounted for rotation to permit selection of said light output wavelengthor spectral band and the second diffraction grating can be mounted forrotation to permit selection of the width of the light output. Inanother embodiment, a motorized computer controlled mounting forrotating the first and/or second diffraction gratings can be utilized.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages, and the system and apparatus of the present inventionwill be understood from the following description taken together withthe drawings, in which:

FIG. 1 is a perspective schematic view of an embodiment of the presentinvention illustrating the orientation and relation of the partsthereof;

FIG. 2 is a representation of the transmission display mediumillustrating a resultant spectral transmission;

FIG. 3 is an illustration of the spectral transmission;

FIG. 4 is an illustration of a top view of the embodiment of the presentinvention; and

FIG. 5 is illustration of a side view of the present invention viewedalong line 5—5 of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1-5, the operation of the inventive double gratingthree dimensional spectrograph system 10 may be understood. Generally,system 10 comprises a light source such as fiber optic bundle 12, anentrance slit 14, a first concave grating 16, a middle slit 18, a secondconcave grating 20, and a measurement or recording system 32 FIG. 2.System 32 receives an output spectra illustrated in greater detail inFIG. 3.

Directional references herein, for convenience, are made with respect tothe orientation of spectrograph 10 as shown in the figures, where lightsource 12 is on the right, first grating 16 is on the left, secondgrating 20 is on the right with middle slit 18 disposed between grating16 and grating 20. It is understood that, in practice, spectrograph 10may have any desired orientation.

As can be seen clearly in FIGS. 4 and 5, optic bundle 12, entrance slit14, first concave grating 16, middle slit 18, second concave grating 20,and measurement or recording system 32 have centers which all lie in thesame plane. Fiber optic bundle 12, entrance slit 14, first concavegrating 16, middle slit 18, second concave grating 20, and measurementor recording system 32 are all oriented perpendicular to the plane inwhich they lie. Grating 16 is oriented parallel to grating 20 or at anangle 37 determined by and specific to the grating 16 employed, with theoptical axis of grating 16 parallel or at an angle 37 to the opticalaxis of grating 20. Middle slit 18 is oriented with its edges definingthose points on the spectrum associated with the upper and lowerwavelength ranges of the spectrum produced by grating 16. In accordancewith the preferred embodiment, middle slit 18 is oriented parallel toand on the focal point of the spectrum produced by grating 16, allowingfor easy adjustment of the position and width of the slit to variouspositions on the spectrum by movement along a track or by othermechanical artifice.

Measurement or recording system 32 is oriented to coincide with thefocal plane of grating 20. It may be substantially planar or it may bevaried in configuration to coincide to the shape of the focal planeimaged by grating 20. Alternative recording system orientationsillustrated as phantom lines wherein recording system 36 issubstantially perpendicular to the plane of grating 20. Likewise,recording system 38 can be oriented somewhere between substantiallyplanar and substantially perpendicular. The orientation of recordingsystem 32 being grating dependant.

Spectrograph 10 receives input light to be analyzed from any source,such as a fiber optic bundle 12. Fiber-optic bundle 12 passes lightthrough an optical aperture or slit 14. Light 15 (FIG. 4) exiting theoptical aperture 14 is directed onto first grating 16. The distancebetween the center of grating 16 and the center of slit 14 is determinedby the focal distance of concave grating 16. Grating 16 diffractsincident light 15 to create diffracted light 17. Such diffraction isachieved by the light being dispersed in a first direction. Thedirection of such dispersion is a function of the orientation of thegrooves of the grating in grating 16. This orientation is schematicallyindicated in FIG. 5. In accordance with a preferred embodiment thegrooves of grating 16 are generally parallel to the plane of the paperillustrating the inventive system in FIG. 4. Light 17 passes throughmiddle slit 18. Middle slit 18 selects a spectral segment, outputtingthis light as light 19.

After leaving middle slit 18, light 19 is caused to fall onto secondgrating 20. Second grating 20 is oriented in accordance with thepreferred embodiment of the invention with its grooves roughlyperpendicular to the grooves in first grating 16, as is seen mostclearly in FIG. 4. This results in the dispersion of light 19 into twicediffracted light 21. The orientation of the first and second gratingswith respect to each other results in the dispersion of light in secondgrating 20 being substantially perpendicular to the dispersion of lightby first grating 16. Such orientation need not be perpendicular asvariation will largely change only the orientation of the outputspectra.

First grating 16 and second grating 20 are disposed with their center ofcurvature and optical axes substantially in the same plane. Twicediffracted light 21 is then caused to fall onto measurement or recordingsystem 32.

Measurements or recording system 32 may take any one of a number offorms. For example, it may simply be a sheet of photographic paper, aphotographic negative, or a light sensitive device such as aconventional television tube or an image intensifier. It may also asolid-state device, such as a linear matrix of detectors, or atwo-dimensional array of solid-state detectors as would be employed, forexample, a charge coupled device (CCD).

Light source 12 may be of any suitable type, for example, an emissionsource, a fluorescence source, a Raman source, absorption spectra, orthe like, or even a light source for use in spectroscopy or any otheruse where the characteristics of the light source should be determined.Optical aperture 14 may take any suitable form, such as a slit, as wouldbe employed in spectroscopy and similar applications. In addition,aperture 14 may be provided with radiation in anyone of numerousfashions known in the art, including directly, by a fiber optic bundle,lens, mirror, or other suitable focusing or other light transmittingmeans.

The light 15 input into the system is usually a mixture of light ofdifferent wavelengths, for example, such as are produced by a plasma,hot body radiation, or particles of gas and plasma, but in certain casesit is contemplated, that the invention may be used for calibrating,detecting or measuring substantially monochromatic light. Typical samplesources are industrial materials, such as the steel slag samples, raisedto a sufficient temperature to emit characteristic elemental spectra,light that has been passed through biological or other fluids togenerate an absorption spectra and a wide variety of scientificapplications, typical of those where spectroscopy is employed.

The advantages of the system flow from the fact that the two gratings ofthe system disperse light in different directions, with particularlygood results achieved when such dispersions are at right angles to eachother. The invention may be applied to a wide variety of systems. Forexample, the groove density may be varied depending upon the desiredoperating wavelength of light. If one desires to make a more compactsystem, the gratings may be positioned with their dispersions atsomething less than a right angle.

In use, light 15 to be analyzed is introduced into the system through afiber-optic bundle 12. The emitted light then passes through opticalaperture or slit 14. Slit 14 has a height of 6 mm and an adjustablewidth of less then 2 mm.

Window 33 on slit 14 is positioned at the position recommended by themanufacturer of grating 16 as the source point for light to be analyzed.Slit 14 functions as an input aperture and is configured and dimensionedsuch that the light is directed onto grating 16, which diffracts light15.

Grating 16 is of the type sold under catalog number CP-20 by JY/SPEX. Ithas a concave configuration, with a radius of curvature of 20 mm, adiameter of 8 mm , a groove density of 550, and is normally employedwith a working spectral range of 300-800 nm. The expanded usefulspectral range of this grating using the inventive configuration, asdescribed in detail below, is noted.

Grating 16 produces a spectrum 23 which includes a range of wavelengthsincluding a selected wavelength 25. In spectrum 23 shorter wavelengthsare lower in FIG. 1 and longer wavelengths are higher in FIG. 1.Selected segment 25 in resultant spectrum 23 is then passed by aperture27 in middle slit 18. Aperture 27 as two edges 41 and 43 which arepositioned to coincide with the position of light having a wavelengthcorresponding with the edges of the spectrum to be passed. By “edges” ismeant the shortest wavelength passed by aperture 27 and in the longestwavelength passed by aperture 27. It will be understood that aperture 27passes both the shortest and the longest wavelength and all thewavelengths in between these two wavelengths. Aperture 27 is orientedperpendicular to the direction of dispersion of light by grating 16. Thedistance between the center of window 27 and the center of grating 16 isabout 25 mm. The distance between the center of window 27 of middle slit18 and the center of slit 14 is about 20 mm.

In the preferred embodiment, aperture or window 27 in middle slit 18 hasa height of less than 6 mm. The width of middle slit 18 is 0.05 mm.Although aperture 27 can be greater then 6 mm in height, the quality oflight 19 projected onto second grating 20 would be compromised as wouldthe image output by the system. The height of aperture 27 determines thewavelength range that will be imaged on measurement or recording device32. This is the case because light is dispersed by grating 20 alongdirection 29, as illustrated in FIG. 2, and window 27 has its lengthoriented parallel to direction 29. In spectrum 22 shorter wavelengthsare on the left in FIG. 1 and longer wavelengths are on the right inFIG. 1.

The distance between the center of grating 20 and the center of window27, in accordance with the preferred embodiment is 137 mm. The distancebetween the center of grating 20 and the center of grating 16, inaccordance with the preferred embodiment is 160 mm. The distance betweenthe center of grating 20 and in the center of measurement or recordingdevice 32 is 135 mm. More particularly, aperture 27 is positioned at thepoint recommended by the manufacturer for the position of the aperturewith respect to grating 20.

The width of aperture or window 27 and the input width of the sourcedetermines the final resolution of the output spectra emitted ontomeasurement or recording device 32. The spectral segment is thenprojected onto second grating 20.

Grating 20 is of the type sold by Jobin Yvon under catalog numberCP-140. It has concave configuration with a diameter of 70 mm, radius ofcurvature of 140 mm, a groove density of 230 grooves per/mm, and anormal working spectral range of 380-780 nanometers. The groove densitymay be varied depending upon the desired operating wavelength of light,with closer grooves being used for shorter wavelengths of light. Thediameter of grating 20 must be much larger than the height of middleslit 18 for uniform efficiency for the various wavelengths of light thatmay be projected upon it.

In accordance with the preferred embodiment, grating 16 and/or grating20 is of a type that is aberration correcting as covered under U.S. Pat.Nos. 3,628,849 and 3,930,728.

Alternatively, grating 16 and grating 20 can be aberration correctingplane grating 16 of the type sold by Jobin Yvon under catalog number52300680, and grating 20 of the type sold by Jobin Yvon under catalognumber 52302120. Such plane gratings as are employed in a Czrney Turnermonochrometer.

In accordance with the preferred embodiment, grating 20 is disposed suchthat it disperses light perpendicular to first grating 16. Grating 16and grating 20 have been shown as having a round configuration, othersuitable shapes may be utilized in practicing the invention, as will beapparent to those of ordinary skill in the art. The resultant spectralemission is then projected onto measurement or recording device 32.

Referring to FIG. 2, measurement or recording device 32 is positionedwith the center of measurement or recording device 32 at the distance ofapproximately 135 mm from the center of grating 20. This positioncoincides with the position of the focal plane of grating 20 andmeasurement or recording device 32 is oriented to substantially coincidewith the focal plane. Alternatively, measurement or recording device 32may be shaped to conform to the shape of a focal surface whose shape isnot substantially planar. Measurement or recording device 32 may be madeof any suitable material, such as photographic paper, or may take theform of any suitable device, such as an input device for a spectralanalyzer.

System 10 is designed to have stray light rejection on the order of 10⁻⁵to 10⁻⁶. Second and higher order spectra are outside of the resultantspectrum 22. However, devices and materials specific for this type ofdetection can be utilized to gather spectral information that falls intothe ultraviolet or infrared ranges.

The resultant spectrum 22 (FIG. 3) is a product of the spectral contentof light source 12 to be analyzed. The same is first broken down byfirst grating 16 which disperses the light into a spectrum 23 in thedirection indicated by arrow 35. Spectrum 23 is formed by diffractedlight 17. The wavelength range to be selected is determined by aperture27 of middle slit 18.

The selected light 19 output by aperture 27, after being diffracted asecond time by grating 20 is dispersed in the direction of arrow 29 ontomeasurement or recording device 32, where it forms an output spectrum30.

The content of spectrum 30 may be understood from FIG. 3. Spectrum 30comprises a range of wavelengths λ1 through λn within the selectedspectral range.

The resultant direction of dispersion of light is illustrated by arrow26. Referring back to FIG. 2, the tangent of angle α is determined bythe formula:tan α=D ₁ /D ₂,where D₁ is the dispersion of grating 16/and D₂ is the dispersion ofgrating 20.

The spectroscopic system 10 of the present invention is designed tofunction analyzing light in the ultraviolet wavelengths, down to awavelength of approximately 180 nm, into the infrared range towavelengths on the order of 10 um. Devices currently in use aregenerally limited to the range of 400-700 nm, though some devicespresently on the market can function as low as 350 nm with the use ofspecial filters.

The inventive spectrograph 10 can be contained in a light tight housingwhich excludes substantially all stray light from the housed opticalelements. The housing should be robust and durable to protect the opticsduring use and be sufficiently rigid to maintain the proper geometricrelationship between the elements. The housing can be internallyconfigured to support gratings 16 and 20 for rotation, and shaped toaccommodate such rotation. The inside of the housing is painted with aflat black paint, which probably has a relatively rough surface.Additionally, the housing can have a suitable opening or openings forlight source 12 and connections to transmission measurement or recordingdevice 32 and may be provided externally with mounting structures suchas threaded extensions, bosses, recesses or aperture flanges forattachment to other modules or equipment.

If desired, a motorized and optionally computer-controlled mounting canbe provided for rotating grating 16 to automate the scanning ofavailable wavelengths used to provide the absorptive information of thesample under observation. Such automatic rotation is performed in orderto change the range of wavelengths detected by the detector 32 whilekeeping the resolution of the spectrum. In order to detect the samerange of wavelength, rotation of grating 16 and grating 20 should besynchronized.

Likewise, a motorized and optionally computer-controlled mounting can beprovided for rotating grating 20 to automate the scanning of the outputwavelengths.

Further, a motorized and optionally computer-controlled mounting can beprovided for moving slit 18 in the X-axis, Y-axis, and/or Z-axis, toautomate the scanning of available wavelengths used to provide theabsorptive information of the sample under observation. Grating 16 andgrating 20 remaining stationary. Such automatic motion is performed inorder to change the range of wavelengths detected by the detector 32while keeping the resolution of the spectrum.

Likewise, a motorized and optionally computer-controlled mounting can beprovided for rotating grating 16 and/or grating 20 to automate thescanning of the output wavelengths.

While an illustrative embodiment of the invention has been described, itis, of course, understood that various modifications of the invention,with the benefit of this disclosure, will be obvious to those ofordinary skill in the art. Such modifications are within the spirit andscope of the invention which is limited and defined by the appendedclaims.

1. A device for separating light into its components providing a lightoutput in a spectral band from sample light input into the device,comprising: (a) an input light source; (b) a first concavespectrographic diffraction grating positioned to receive light from saidinput light source and configured to provide a once diffracted lightoutput dispersing the components of said input light source, saiddispersing forming said input light into an intermediate spectra, saidintermediate spectra being formed in a focal volume by said oncediffracted light; (c) a slit substantially positioned in said focalvolume or approximately in said focal volume, said slit having a firstedge whose position corresponds to the shortest wavelength for light tobe passed by said slit and having a second edge whose positioncorresponds to the longest wavelength for light to be passed by saidslit; and (d) a second concave diffraction grating positioned to receivesaid once diffracted light from said slit and configured to provide animage on a focal surface in the form of a twice diffracted light output,said first concave spectrographic diffraction grating being oriented todisperse the components in a first direction on said focal surface as afunction of wavelength, said second concave diffraction gratingdispersing the components of said once diffracted light in a seconddirection on said focal surface, said second direction on said focalsurface being different from said first direction on said focal surface,said dispersion forming said input light into an output spectra, saidoutput spectra being formed on said focal surface by said twicediffracted light, said output spectra having an orientation which isoblique with respect to said first and second directions.
 2. A devicefor separating light into its components as in claim 1, furthercomprising a movable member supporting said slit at a plurality ofpositions with said first and second edges at positions corresponding tothe shortest and longest wavelength in a range of wavelengths to beselected by said slit.
 3. A device for separating light into itscomponents as in claim 1, further comprising a measuring devicepositioned substantially along the focal surface of said second concavediffraction grating.
 4. A device for separating light into itscomponents as in claim 3, wherein said measuring device is a solid-statedetector array.
 5. A device for separating light into its components asin claim 1, further comprising a rotary mounting for rotatablysupporting said first concave diffraction grating.
 6. A device forseparating light into its components as in claim 1, further comprising arotary mounting for rotatably supporting said second concave diffractiongrating.
 7. A device for separating light into its components as inclaim 1, further comprising a first rotary mounting for rotatablysupporting said first concave diffraction grating and a second rotarymounting for rotatably supporting said second concave diffractiongrating.
 8. A device for separating light into its components accordingto claim 1 wherein said input light source comprises a fiber opticbundle disposed at said light aperture.
 9. A device for separating lightinto its components according to claim 1 wherein said second grating hasa diameter at least two times greater then said height of said slit. 10.A device for separating light into its components according to claim 1wherein said first and second grating have a diameter of 3 mm or higher.11. A device for separating light into its components according to claim1 wherein said first and second directions are substantiallyperpendicular to each other.
 12. A device for separating light into itscomponents according to claim 1 operable at wavelengths in a range of180 nm to 10 um.
 13. A device for separating light into its componentsaccording to claim 1 wherein stray light rejection is in the order of10⁻⁵ to 10⁻⁶.
 14. A device for separating light into its componentsaccording to claim 1 wherein said first and second grating have a groovedensity of 80/mm or higher.
 15. A device for separating light into itscomponents according to claim 1 wherein said second direction istransverse to said first direction.
 16. A device for separating lightinto its components according to claim 15 wherein the width of said slitdetermines the range of a spectral segment.
 17. A device as in claim 1,wherein said second direction has a vector component that isperpendicular to said first direction.
 18. A device for separating lightinto its components providing a light output in a spectral band fromsample light input into the device, comprising: (a) an input lightsource; (b) a first concave spectrographic diffraction gratingpositioned to receive light from said input light source and configuredto provide a once diffracted light output dispersing the components ofsaid input light source, said dispersion forming said input light intoan intermediate spectra, said intermediate spectra being formed in afocal volume by said once diffracted light; (c) a slit substantiallypositioned in said focal volume or approximately in said focal volume,said slit having a first edge whose position corresponds to the shortestwavelength for light to be passed by said slit and having a second edgewhose position corresponds to the longest wavelength for light to bepassed by said slit; and (d) a second concave diffraction gratingpositioned to receive said once diffracted light from said slit andconfigured to provide an image in the form of a twice diffracted lightoutput, said first concave spectrographic diffraction grating beingoriented to disperse the components in a first direction on a focalsurface as a function of wavelength, said second concave diffractiongrating being oriented to disperse the components of said oncediffracted light in a second direction, said second direction having avector component orthogonal to said first direction, said dispersionforming said input light into an output spectra, said output spectrabeing formed in said focal surface by said twice diffracted light.